Encoders convert angular position into electrical signals and can employ different ways to detect angular or rotary motion such as mechanical means utilizing brush contacts or magnet/inductive methods. But the most common, reliable and widely used devices are non-contact optical receptors employed by optical encoders. Optical encoders are generally of two different types: incremental and absolute position. Incremental-type encoders are the simpler of the two and are either of the reflective or transmissive type.
Reflective optical encoders are comprised of three basic active elements: a light source, a photo-detector and a reflective code disc. These three basic parts along with a few other passive components generate a series of electrical square wave pulses as a code disc mounted on a shaft rotates past the light source and detector on the encoder module chip. The number of lines or bars on the disc determines an incremental encoder's resolution during one revolution. The evenly-spaced opaque radial lines on the reflective surface of the disc cause interruptions that act as a shutter to alternatively open and close the light path from the light source to the photo detector. The electronic signal generated by light falling on the photo detector is amplified within the encoder module and converted to a square wave with “high” and “low” values corresponding to light present or absent at the photocell, respectively. The span of one “high” and one “low” is known as one full cycle or 360 electrical degrees.
In addition to shaft position, rotary encoders can also indicate speed and direction of motion. Electronics integral within the encoder module generates two separate channels that work together. The two channels are in quadrature, or out of phase by 90 electrical degrees. This offset of the two separate waveforms produces four distinct states within each electrical cycle.
When used for electric motor commutation, the incremental rotary encoder is typically assembled directly to the motor's back end on an extension of the motor shaft. Another method is to attach the encoder directly to a load's rotating shaft or member. The shaft position encoder is only one part of the circuit which, along with receiving electronics such as counters or controllers, completes the motor control circuit.
FIG. 1 depicts an example of a basic, PC board based reflective optical incremental rotary encoder assembly. The components are shown mounted on the back end of a fractional horsepower motor. The reflective code disc is mounted to the hub with the artwork including reflective and non-reflective bars facing downward toward the encoder module. The encoder module chip is surface-mounted or plug-in mounted to the PC board assembly which is in turn mounted to the encoder base and motor. The hub is then assembled to the motor shaft with a small gap between the code disc and the encoder module. In most cases, an output connector is mounted to the PC board joined with wire leads or cable (not shown) exiting the cover.
Prior art encoders require separate and different components to match each different resolution. There is therefore a need in the art for standardized components which have the versatility to be reconfigured to provide different resolutions without requiring additional parts.